The Lithuanian–Soviet War or Lithuanian–Bolshevik War (Lithuanian: karas su bolševikais) was fought between newly independent Lithuania and the Russian Socialist Federative Soviet Republic in the aftermath of World War I. It was part of the larger Soviet westward offensive of 1918–1919. The offensive followed the retreat of German troops and sought to establish Soviet republics in Ukraine, Belarus, Lithuania, Latvia, Estonia, and Poland, and to link up with the German Revolution. By the end of December 1918, Soviet forces reached Lithuanian borders. Largely unopposed, they occupied one town after another and by the end of January 1919, they controlled about two thirds of the Lithuanian territory. In February, the Soviet advance was stopped by Lithuanian and German volunteers, who prevented the Soviets from capturing Kaunas, the temporary capital of Lithuania. From April 1919, the Lithuanian war went parallel with the Polish–Soviet War. Poland had territorial claims over Lithuania, especially the Vilnius Region; these tensions spilt over into the Polish–Lithuanian War.

British-Polish historian Norman Davies summarized the situation: "the German army was supporting the Lithuanian nationalists, the Soviets were supporting the Lithuanian communists and the Polish Army was fighting them all." In mid-May, the Lithuanian army, now commanded by General Silvestras Žukauskas, began an offensive against the Soviets in Northeastern Lithuania. By mid-June, the Lithuanians reached the Latvian border and cornered the Soviets among lakes and hills near Zarasai, where the Soviets held out until the end of August 1919. The Soviets and Lithuanians, separated by the Daugava River, maintained their fronts until the Battle of Daugavpils in January 1920. As early as September 1919, the Soviets offered to negotiate a peace treaty, but talks began only in May 1920. The Soviet–Lithuanian Peace Treaty was signed on July 12, 1920. Soviet Russia fully recognized independent Lithuania.

Lithuania became part of the Russian Empire after the final partition of the Polish–Lithuanian Commonwealth in 1795. During World War I, Lithuania was occupied by Germany and made part of Ober Ost. On February 16, 1918, the Council of Lithuania declared independence from both Germany and Russia. Three weeks later, the Bolsheviks, encumbered with the Russian Civil War, sued for peace with the Central Powers and signed the Treaty of Brest-Litovsk. They renounced Russian claims to Finland, Estonia, Latvia, Ukraine, Lithuania, and Poland. However, Germany granted the Lithuanians only minimal autonomy and they could not establish de facto independence. That changed when Germany lost the war and signed the Compiègne Armistice of 11 November 1918. Lithuania soon began organizing basic institutions and established its first government led by Augustinas Voldemaras.

On November 13, 1918, the Soviet Russian government renounced the Treaty of Brest-Litovsk, which had assured Lithuania's independence. The Bolshevik Western Army followed retreating German troops, maintaining a distance of 10–15 kilometres (6.2–9.3 mi) between the two armies. Demoralized Germans often left valuable armaments and other equipment to the Soviets. The Soviets attempted to spread the global proletarian revolution and sought to establish Soviet republics in the region. They saw Baltic states as a barrier or a bridge into Western Europe, where they could join the German and the Hungarian revolutions. By the end of December 1918, Bolshevik forces reached eastern Lithuania.

Augustinas Voldemaras, the first Prime Minister of Lithuania, did not believe that forming the military was a priority and advocated for Lithuanian neutrality. He trusted that German mercenaries would protect Lithuania until the upcoming Paris Peace Conference could establish peace. Residents organized local self-defense units to defend themselves from the retreating Germans. The first laws regarding the army were not issued until 23 November 1918. Some Lithuanians, who had served in the Russian army during the World War, returned to Lithuania and started organizing battalions in Kaunas, Gardinas, and Alytus. They lacked guns, ammunition, and officers.

At the end of December, with the Bolsheviks already in the country, Lithuania was left leaderless. Augustinas Voldemaras, Chairman of the Council of Lithuania Antanas Smetona, and Martynas Yčas, minister of finance, departed for Germany to ask for financial assistance. General Kiprijonas Kundratavičius, vice minister of defense, suggested a retreat to Gardinas and refused to command the Lithuanian defense. The first cabinet of ministers resigned on December 26, 1918. Mykolas Sleževičius stepped in and organized a new government. On December 29, he issued the first mass appeal in four languages calling for volunteers for the Lithuanian Army. Sleževičius' government adopted a new policy on land reform, which could be summarized in a slogan "land for those who cultivate it." It meant the land would be taken from large landowners and redistributed first to the volunteers for free and then to small peasants for a fee. Mobilization of officers was announced only on January 25; about 400 people responded.

In Berlin, Smetona and Yčas signed a loan agreement with Germany for 100 million marks. The money was used primarily to build and supply the army. They further negotiated direct German support in the war against the Soviets. Article 12 of the Compiègne Armistice required the Germans to protect Lithuania from possible Soviet attacks, but Germany was also interested in maintaining its influence in the region and weakening Russia. At first, they tried to organize volunteers from the retreating soldiers of the 10th German Army, commanded by General Erich von Falkenhayn. However, the soldiers were tired and demoralized and wanted to return home as soon as possible. Recruitment continued in Germany, especially in Saxony. The volunteers were paid 30 marks per month plus 5 marks per day and had to sign up for three months. The first Saxon volunteers, as they became known, arrived in Kaunas at the beginning of January, but quite a few of them were judged unfit for duty and sent back. By the end of January, German volunteers numbered 4,000. They were unreliable, as the German Revolution increased the popularity of the Spartacus League and Soviet causes. There were several attempts at a coup against the Lithuanian government. These volunteers were stationed in and around Kaunas: Alytus, Jonava, Kėdainiai, and Baisogala.

At first, they were organized into the 46th Saxon Volunteer Division. On February 22, Lieutenant-General Walter von Eberhardt became its commander. In April–May, German forces were reorganized into the South Lithuania Volunteer Brigade, composed of three regiments, (18th, 19th, and 20th) and a separate battalion in Raseiniai. The 18th Regiment fought alongside Lithuanians; the 19th Regiment guarded the Kaunas area and did not participate in battle; the 20th Regiment was stationed in Gardinas and then in Kėdainiai; the separate battalion joined the Bermontians. The Baltische Landeswehr, led by General Rüdiger von der Goltz, organized a coup against the Latvian government and captured Riga. On May 23, the Paris Peace Conference, reacting to these events, asked Germany to withdraw its troops from both Latvia and Lithuania as soon as local forces could defend themselves. The last Saxon Volunteers left Lithuania in mid-July.